Radio frequency identification (“RFID”) has been proposed for use in a number of applications in which an RFID tag is attached to an item and subsequently interrogated or read to obtain information regarding that item. For example, U.S. Pat. Nos. 6,232,870 and 6,486,780, and PCT Publication WO 00/10122 describe various functions and applications for RFID systems, and exemplify the use of RFID tags in libraries. U.S. Pat. No. 5,963,134 also describes certain uses for RFID systems in libraries and for other applications.
The design of a typical RFID tag reflects its origin in the semiconductor and printed circuit board industries. Although functional, the design has a number of features that increase the cost of the finished article and efficiency especially at ultra high frequencies (“UHF”). In a resonant RFID tag, the electrical inductance of an antenna is connected in parallel with a capacitor such that the resonant frequency of the thus-formed circuit is tuned to a prescribed value. In more advanced forms, the circuit of the RFID tag may include an integrated circuit electrically and mechanically bonded to the antenna on a substrate, wherein the voltage induced in/on the antenna by a reader signal provides power to operate the integrated circuit.
Various methods have been developed to design RFID tags, such as disclosed in the following references: U.S. Pat. No. 6,501,435; U.S. Pat. No. 6,100,804; and PCT Publication WO 00/26993.